1. Field of the Invention
The present invention relates to an apparatus for measuring a blood pressure value of a living subject while the pressure of an inflatable cuff being wound around a body portion of the subject is changed.
2. Related Art Statement
There is known a linear-pressure-increase blood-pressure (BP) measuring apparatus which measures a BP value of a living subject based on a pulse wave which is produced from an artery of the subject in synchronism with the heartbeat of the subject while the air pressure of an inflatable cuff being wound around a body portion of the subject is increased linearly at a predetermined rate. This BP measuring apparatus is disclosed in Laid-Open Publication No. 59(1984)-129051 of Unexamined Japanese Patent Application, for example.
The above BP measuring apparatus enjoys the advantage that it can quickly deflate the cuff immediately after determining a systolic BP value of the subject. Thus, the BP measuring apparatus need not increase, in advance, the pressure of the cuff up to a predetermined target value which is sufficiently higher than a systolic BP value of the subject. Therefore, the cuff pressure is not increased up to an unnecessarily high value, or the measurement time is not unnecessarily increased. This leads to reducing the discomfort the subject feels due to the pressing of the cuff.
The prior BP measuring apparatus detects a pulse wave, i.e., pressure oscillation which is propagated to the cuff in synchronism with the heartbeat of the subject while the cuff pressure is increased at a predetermined low rate, and determines one or more BP values of the subject based on variation of respective amplitudes of heartbeat-synchronous pulses of the pulse wave. The increasing of the cuff pressure is effected by operating an air pump to supply pressurized air to the cuff. Thus, the pulsation of the pressurized air resulting from the operation of the air pump is mixed as noise with the pressure oscillation detected from the cuff.
Meanwhile, the pulse wave that is propagated from the subject to the cuff has frequencies within a specific range. Therefore, a band-pass filter having a frequency characteristic corresponding to the specific frequency range of the pulse wave is advantageously used for removing the noise from an electric signal representative of the pressure oscillation in the cuff.
On the other hand, a BP measuring apparatus is required to be able to implement a common pressure-decrease BP-measure mode in which the cuff pressure is quickly increased up to a predetermined target value higher than a systolic BP value of a subject and subsequently is decreased at a predetermined low rate and, based on a pulse wave detected as pressure oscillation propagated to the cuff during this slow cuff-pressure decreasing, one or more BP values of the subject are determined. Thus, the BP measuring apparatus employs a high-power air pump which can increase the cuff pressure up to the target value in a short time. Therefore, in a pressure-increase BP-measure mode in which a BP value is measured based on a pulse wave detected while the cuff pressure is linearly increased, a driving or rotating member of the air pump must be rotated at a reduced speed. In this case, the frequency of the pulsation of pressurized air due to the operation of the air pump approaches the specific frequency range of the pulse wave, and a band-pass filter cannot effectively remove the noise resulting from the pulsation of pressurized air, from the pressure oscillation detected from the cuff.
There is known another BP measuring apparatus including an air pump which outputs a pressurized air and supplies the pressurized air to an inflatable cuff to increase quickly an air pressure in the cuff up to a predetermined target value before one or more BP values of a living subject is/are determined. The cuff includes a rubber bag which is inflatable when being supplied with the pressurized air from the air pump. The cuff is wound, in advance, around a body portion of the subject to press an artery of the body portion. The initial volume of the cuff when the cuff is initially wound around the body portion will change depending upon the winding or fastening force which a doctor or a nurse applies to the cuff. Therefore, when the air pump is operated according to a predetermined output power, the rate of increase of the cuff pressure will change depending upon the initial volume of the cuff. Hence, it is recommended that the cuff be wound or fastened such that three fingers can be inserted between the cuff and the body portion. However, the body portion such as an upper arm around which the cuff is wound easily deforms. Thus, the initial volume of the cuff easily changes because of the different manners of winding of the cuff or the different thicknesses or diameters of respective body portions.
In the second BP measuring apparatus, if the cuff is wound around the body portion with a high winding force, the initial volume of the cuff becomes small, and if the winding force applied to the cuff is low, the initial cuff volume becomes large. In the first case, the pressure-increase rate of the cuff becomes high, and the cuff pressure may exceed the target value by more than a permissible amount even though the air pump is stopped at the time of detection of the target value. Since the cuff pressure is decreased from the excessively high pressure level, the body portion is pressed under the high cuff pressure for a longer time. Thus, the discomfort the subject feels increases. In the second case, the pressure-increase rate of the cuff becomes low, and the time needed to reach the target value accordingly increases. Thus, the discomfort of the subject also increases.